Looping query over multiple rows - mysql

Question

I need to scan two tables (all rows where the name matches), and then I need to find the MAXIMUM difference in the sell and buy prices. (highest profit that could be made)
How can I find this result strictly using mysql? I want the query to perform the calculation.
I have two tables:
SELL:
Name | Sell_price | Seller
------------------------
Toyota | 12,000 | Mike
Toyota | 11,000 | Tony
Toyota | 9,000 | James
----------------------------
Mazda | 5,000 | Craig
Mazda | 4,000 | Roger
Mazda | 3,000 | Jones
Buy:
Name | Buying_Price | Buyer
------------------------
Toyota | 13,000 | Steve
Toyota | 12,000 | Andy
Toyota | 10,000 | Charles
----------------------------
Mazda | 5,000 | Thatguy
Mazda | 4,000 | Dog
Mazda | 3,500 | Cat
Result:
Name |Profit | Buyer | Seller
----------------------------------
Toyota | 2,000 | Charles | Mike
---------------------------------
Mazda | 1,500 | Craig | Cat

Answer 1

Join the tables on brand and then filter to only get the maximum sell and minimum buying prices:
select
sell.name,
sell.sell_price - buy.buying_price as profit,
buy.buyer,
sell.seller
from sell
join buy on buy.name = sell.name
where (sell.name, sell.sell_price) in
(
select name, max(sell_price)
from sell
group by name
)
and (buy.name, buy.buying_price) in
(
select name, min(buying_price)
from buy
group by name
);
Or do it vice versa, filter first and then join:
select
s.name,
s.sell_price - b.buying_price as profit,
b.buyer,
s.seller
from
(
select *
from sell
where (name, sell_price) in
(
select name, max(sell_price)
from sell
group by name
)
) s
join
(
select *
from buy
where (name, buying_price) in
(
select name, min(buying_price)
from buy
group by name
)
) b on b.name = s.name;

Answer 2

You need to compare every row against every other row, so OUTER JOIN is your friend. Then you can do a calculation in your SELECT and use AS to assign a temporary column name:
SELECT s.name, (s.sell_price - b.buy_price) AS profit, s.seller, b.buyer
FROM `sell` as s
OUTER JOIN `buy` as b ON s.name=b.name
To then sort it, you simply need to utilize ORDER BY:
SELECT s.name, (s.sell_price - b.buy_price) AS profit, s.seller, b.buyer
FROM `sell` as s
OUTER JOIN `buy` as b ON s.name=b.name
ORDER BY profit
You are probably right not to do this in code, depending on your context.

Answer 3

Something like (not tested):
Select s.sell_price - b.buyer_price from seller as s join buyer as b on s.name like b.name group by s.name, b.name;

Answer 4

A little bit modified query but I suppose that it's giving more precise result.
If You are making calculating of your proffit as a seller, then You have to select only rows where the buying price is greater then the selling price.
SELECT s.name, s.sell_price, b.buy_price, (b.buy_price - s.sell_price) AS proffit, s.seller, b.buyer
FROM `sell` as s
RIGHT JOIN `buy` as b ON s.name=b.name WHERE b.buy_price > s.sell_price ORDER BY proffit DESC
and the result will be:
name | proffit | seller | buyer
--------------------------------------------
Toyota | 4,000 | James | Steve
Mazda | 2,000 | Jones | Thatguy
Toyota | 1,000 | James | Charlie

Answer 5

According to your result table, u need to retrieve only records which have profits without any damages. You can try like this one.
SELECT s.Name, (s.Sell_Price-b.Buying_Price) AS Profit, b.Buyer, s.Seller
FROM Sell s RIGHT JOIN Buy b ON s.Name=b.Name
WHERE b.Buying_Price > s.Sell_Price;